Unfortunately, many biopolymers are sensitive to common sterilization procedures, e.g., heat sterilization. Heat sterilization can often lead to pronounced changes in the physico-chemical properties of the biopolymer such that the resulting sterile biopolymer is rendered unsuitable for its intended use.
Sterilization methods that are currently applied to medical materials include, for example, heat treatment, high-pressure vapor sterilization (e.g. autoclave sterilization), ethylene oxide gas (EOG) sterilization, supercritical carbon dioxide sterilization and radiation sterilization. See for example, U.S. Pat. No. 6,891,035, U.S. Pat. No. 6,149,864, U.S. Pat. No. 5,621,093, U.S. Pat. No. 4,263,253, US 2006/0292030 and US 2007/0009578. Available sterilization methods are typically assessed in relation to the robustness of the particular biopolymer to be sterilized. For example, high-pressure vapor sterilization can be used for a biopolymer only to the extent that the biopolymer can endure high temperatures and high pressures. However, very few biopolymers including hyaluronic acid can endure such high temperatures and high pressures. EOG sterilization could be useful because the process suppresses the thermal breakdown of the biopolymer. However, the presence of residual ethylene oxide in the composition can be problematic.
Hyaluronic acid is a non-immunogenic substance and because of its viscoelastic and hydrophilic properties hyaluronic acid has been used for many years as an eye vitreous or joint fluid replacement or as a supportive medium in ophthalmic surgery, as disclosed for example in U.S. Pat. No. 4,141,973 of Balazs. In joint fluids, the hyaluronic acid solution serves as a lubricant to provide a protective environment to the cells, and for this reason, it is used in the treatment of inflamed knee joints. EP-A-0 698 388 of Chemedica S. A. discloses an ophthalmic preparation for use as artificial tears containing hyaluronate as a viscosity thickener. The consumer use of products that include hyaluronic acid requires the manufacturer to sterilize the consumer product, and if used as an open multi-dose formulation, an additional step must be taken to preserve the formulation product.
Hyaluronic acid is one biopolymer known to be very sensitive to thermal sterilization processes. Heat sterilization of hyaluronic acid is known to accelerate the hydrolysis or oxidation of hyaluronic acid, thereby causing a significant and often detrimental decrease in the average molecular weight of the biopolymer. For many pharmaceutical applications, a relatively low molecular weight form of hyaluronic acid in the formulation is not desirable. Typically, the low molecular weight forms of hyaluronic acid do not provide the desired rheological properties of the high molecular weight form of hyaluronic acid. To compensate for the breakdown of the hyaluronic acid in the aforementioned heat sterilization methods, one could possibly begin with a hyaluronic acid with a higher molecular weight than desired. This accommodation, however, leads to process inefficiencies because the product yield of hyaluronic acid decreases as the average molecular weight of the biopolymer increases.
Another known method of sterilizing hyaluronic acid is by filtration. Common filtration processes are used in industrial processes for preparing purified hyaluronic acid salts in a concentrated form, usually in the form of dried powder, whereby an aqueous solution is passed through the filter and subsequently dried. Such a filtration processes is described in European patent application EP 867453 and WO 00/44925. U.S. Publication No. 2006/0052336 describes a method of filter sterilizing a high molecular weight hyaluronic acid at a relatively low hyaluronic acid concentration to reduce the viscosity of the solution. Following the filtration step, the concentration of the hyaluronic acid in the solution is concentrated by applying a vacuum and removing the water to obtain a desired more concentrated form of hyaluronic acid.
U.S. Pat. No. 5,093,487 describes a method of preparing an aqueous solution containing a high molecular weight form of hyaluronic acid (preferably greater than 1.2 MDa) that can be filtered more easily by reducing the viscosity of the solution. This reduction in viscosity of the solution is said to be possible by a “controlled heat treatment” or by “filtration of a minimum concentration solution with high pressure or vacuum through small pore membranes.” These viscosity reducing processes also lead to a hyaluronic acid form that maintains its lower viscosity over time, i.e., the induced reduction in viscosity is stated as being irreversible. In particular, the heat treatment of a 1 wt. % hyaluronic acid solution having an initial weight average molecular weight of 1.1 MDa at different heating temperatures (30° C., 40° C. and 50° C.) over a one month treatment period is described. For each hyaluronic acid solution no reduction of the weight average molecular weight is reported. The '487 patent also reports that the heat treated solutions of hyaluronic acid can be filtered through a 0.22 micron pore size filter required for sterilization.
Blumberg et al., in Changes in Physical Characteristics of Hyaluronate, pp. 1454-61, reports on changes in the physical characteristics of hyaluronic acid solutions as a function of sodium chloride concentration. With respect to viscosity, an increase in the ionic strength of the solution results in a decrease in viscosity. The decrease in viscosity is observed upon the addition of relatively small amounts of salt. “After approximately 0.5 M [NaCl], the drop [in viscosity] is gradual and after 1.3 M, there is very little change with the addition of even large amounts of salt.” Id. at 1456. In other words, the viscosity is very sensitive to changes in sodium ion concentration just below that of physiological saline, and beyond a “sodium ion strength of approximately 0.15, the hyaluronate is relatively insensitive.” Id. at 1459.
Based upon the foregoing, it can be seen that the sterilization of aqueous biopolymer solutions, and in particular, aqueous solutions of hyaluronic acid, is not at all routine, and there are numerous technical challenges to arrive at a substantially non-degraded sterile solution. These technical challenges are magnified if a commercial-scale production batch is the objective.